Where can I sign up?

http://www.wikispaces.com/site/for/teachers100K

Wikispaces is making 100,000 wikispaces available to teacher for FREE!  These spaces can be private -- only those who are invited can view the wikispaces.

TeachersFirst has created a handy Wiki Walk-Through which steps you through using wikis in education in a clear and easy to follow way.
 

Pedro has more information for you.